Color matching of powder coating compositions has been carried out, for example, by the following method to realize the color specified by the user: coloring pigments, powder resin, curing agent, etc. are dry blended, and the obtained blend is mixed and melted using a twin-screw dispersing device or the like, followed by cooling, coarse grinding, fine grinding, and sieving to produce contemplated color-matched powder coating compositions.
The above-mentioned production of color-matched powder coating compositions poses no problem in producing a large amount of powder coating composition with a specified color, but when powder coating compositions of various colors are produced in small amounts, i.e., in the case of small-lot production of powder coating compositions with different colors, many problems are encountered, including a great loss of powder coating compositions, considerable labor, increased contamination, and so on.
In order to solve such problems, WO 90/06345 discloses a method for color matching a powder coating composition, in which a mixture of two or more kinds of powder coating materials with different colors is granulated using an aqueous acrylic resin as a granulating agent, to obtain a color-matched powder coating composition. Further, Japanese Unexamined Patent Application Publication No. 1995-188586 discloses a color matching method in which two or more kinds of powder coating materials are dry blended for color matching, and the blend is granulated using a synthetic resin as a binder. However, these known methods requires the use of an aqueous acrylic resin and a binder resin both having a high molecular weight to prevent the degradation of film properties, causing problems such as a decrease in film properties such as film smoothness, film appearance, etc. due to the low thermal flowability of the granulated powder coating composition; irregular granules due to high viscosity of the resin components; etc.
Furthermore, Japanese Unexamined Patent Application Publication No. 1999-241039 discloses a color-matched thermosetting powder coating composition obtained by wet-mixing a coloring liquid with an initial-color thermosetting powder coating material. However, since the color-matched powder coating composition is produced by mixing the coloring liquid with one kind of powder coating material, the hue of the resulting powder coating composition greatly depends on the hue of the coloring liquid. Therefore, to obtain a powder coating composition having a greatly different hue from that of the initial-color powder coating material, the coloring liquid needs to be used in a large amount. This may reduce the blocking resistance of the resulting powder coating composition, thereby deteriorating film properties such as film appearance, weather resistance, water resistance, etc.